HIV neurovirulence has been linked to viral tropism for macrophages/microglia (MG);MG represent bone marrow-derived cells of macrophage lineage residing in the central nervous system (CNS). In a series of studies performed by Watry et al. (1995), a neuropathogenic simian immunodeficiency virus (SIV) strain was isolated by serial passage of SIVmac251 through infected MG;after 3 such passages, the progeny SIVmac182 reliably induced CNS disease in rhesus monkeys (RM) within a few months. Based upon these data, we postulate that the ability of a primate lentivirus to cross the blood brain barrier and to replicate efficiently in MG represents a hallmark of lentiviral neurovirulence. We further postulate that the envelope gene of HIV-1 that has been adapted in vitro to human MG will confer neurovirulence to a simian-human immunodeficiency virus (SHIV) encoding such an env gene. We seek to test these hypotheses in the RM model. To date, no MG-adapted SHIV strain has been described. We have inserted env of a primary, MG- adapted R5 HIV-1 strain into our unique SHIV backbone that contains extra NF-kB sites in the long terminal repeats (LTRs), which boosts viral replicative capacity and responsiveness to tumor necrosis factor (TNF)-?. The source for HIV-1 env was a strain isolated during acute infection and then serially passaged through cultured human MG, yielding an R5 progeny virus that grew exceptionally well in MG and gained the ability to induce syncytia. Our novel SHIV construct, SHIV-Bo159N4, replicated well in peripheral blood mononuclear cells (PBMC) of all RM donors tested, maintained the exclusive R5 tropism and high fusogenicity in RM PBMC and induced syncytia in MG. Remarkably, this new SHIV replicated very well in RM, where it induced high peak plasma viral RNA loads and could be detected in cerebrospinal fluid (CSF) within 1 week post- inoculation. Our overall goal is use our novel R5 SHIV in RM to: Aim 1: Test the hypothesis that strong MG tropism of HIV-1 Env will translate into neurovirulence in RM infected with a SHIV encoding the corresponding env gene. Aim 2: Increase SHIV neurovirulence by serial passage through infected MG in rhesus macaques. SHIV-infected animals will be followed prospectively for viral, hematological and neurological parameters. These R21 studies will give new insights into mechanisms of HIV-associated dementia by establishing a direct link between HIV-1 Env MG tropism and neurovirulence while establishing a novel, biologically relevant primate model. PUBLIC HEALTH RELEVANCE: Human immunodeficiency virus type 1 (HIV-1) not only destroys the body's immune system, but can also attack the brain and cause severe neurological damage. Most HIV-1 strains that invade the brain enter cells through a receptor molecule called CCR5 and proliferate efficiently in microglia. These specialized cells reside in the brain and are related to macrophages. We have generated a hybrid virus from the monkey AIDS virus by inserting the envelope gene of an HIV-1 strain that has been adapted to grow efficiently in microglia. Using this new virus as a research tool, we seek to study the role that the HIV-1 envelope gene plays in causing neurological damage in primate models. Our work will provide important information for developing a better treatment strategies aimed at protecting the brain from HIV-1 attack.